Liquid crystal displays are commonly used as display devices for compact electronic apparatuses, because they not only are very thin but also provide good quality images with little power consumption. The liquid crystals in a liquid crystal display do not emit any light themselves. The liquid crystals have to be lit by a light source so as to clearly and sharply display text and images. Thus, a backlight module for an LCD is generally needed.
FIG. 11 shows a schematic, top view of a conventional LCD 100. The LCD 100 includes a flexible printed circuit board (FPC) 110, a liquid crystal display panel 120, a plastic frame 130, two light emitting diodes (LEDs) 140 cooperatively serving as a light source, and a light guide plate 150.
The FPC 110 connects with the liquid crystal display panel 120, and includes a main circuit area 111 and a light source setting area 112. The light source setting area 112 extends from the main circuit area 111. The LEDs 140 are arranged on the light source setting area 112 of the FPC 110. The frame 130 includes a depressed portion 131, and the depressed portion 131 has two openings 132.
The LCD 100 can be assembled according to the following sequence: firstly, setting the light guide plate 150 inside the frame 130; secondly, putting the light source setting area 112 into the depressed portion 131, and placing the LEDs 140 into the openings 132; thirdly, attaching the main circuit area 111 to the frame 130 and the light guide plate 150; and finally, attaching the liquid crystal display panel 120 to the frame 130.
However, the above-mentioned conventional liquid crystal display has the following problems.
The sizes of the openings 132 may not accurately match the sizes of the LEDs 140, due to imprecise manufacturing. When this happens, small gaps exist between the LEDs 140 and the light guide plate 150 after assembly. These gaps may diminish the emitting luminance of the light guide plate 150.
FIG. 12 is a graph showing the negative impact of the above mentioned gaps on the effective utilization of the emitting luminance. X represents the distance between the LEDs 140 and a light incident surface (not labeled) of the light guide plate 150 in millimeters (mm), and Y represents the relative emitting luminance of the light guide plate 150 as a function of X. It can be seen that the greater the value of X, the lower the value of Y. That is, the greater the gap, the lower the value of the relative emitting luminance.
What is needed, therefore, is a backlight module and a liquid crystal display device using the same that overcome the above-described deficiencies.